Light Reactions & Photosynthetic Pigments

LIGHT! Of all the light energy that reaches the earth’s surface, ~5% is transferred to carbohydrates by a leaf Light = a form electromagnetic radiation that travels at 3x10 8 m/s as photons ( – That’s 300,000,000m in ONE SECOND!

LIGHT! Light from the sun is a mixture of photons with different energies When light passes through a spectroscope which contains a prism, photons are separated by energy Most of the photons are invisible to humans but we can see wavelengths of light that range from nm – nm = nanometer = 1 x m (it’s really small) **The Vatican’s chief astronomer invented the spectroscope (1860s)

A Short and Painless History of Photosynthesis Like many people, it was initially thought that plants obtained their ‘food’ from the soil – They do? Don’t they? In the early 1600s, J.B. Van Helmont decided to test this theory – Planted a willow tree in soil and ONLY added water for 5 years After the 5 years, the tree’s mass increased by 75kg The soil decreased by only 60g!! – Thus, the soil was NOT a contributing factor – Thought that it must be water!

A Short and Painless History of Photosynthesis In 1770, Joseph Priestly discovered, by accident, that gases in the air played a role in photosynthesis – Placed a burning candle in a closed container – When it went out, he placed a plant in the container with the candle – After 10 days, he was able to reignite the candle – Thus, proving that plants release a gas into the atmosphere This gas was confirmed as oxygen in 1796

A Short and Painless History of Photosynthesis Jan Ingenhousz, 1796, was the first to realize that sunlight is essential to photosynthesis – Also noted that CO 2 was the source of carbon in plants – But, mistakenly said that CO 2 broke down to produce oxygen “Sunshine splits apart the carbon dioxide molecule that a plant has absorbed in the air; the plant throws out at that time the oxygen alone, and keeps the carbon to itself as nourishment” In 1930, C.B. Van Niel proved that oxygen was produced by water splitting (i.e. Not from CO 2 ) – This was confirmed using isotopes ( 18 O) in 1938

A Short and Painless History of Photosynthesis Light and Photosynthesis In 1905, F.F. Blackman measured the effect in changes in light intensity, [CO 2 ], and temperature on photosynthesis Two main results: 1.At low light intensities, the rate of photosynthesis increased by increasing light intensity but not temperature 2.At high light intensities, the rate of photosynthesis increased by increasing temperature but not light intensity From this study, Blackman concluded that there must be: – a light-dependent reaction (photochemical) – a light-independent reaction (biochemical)

A Short and Painless History of Photosynthesis Blackman then showed that the rate of photosynthesis is sensitive to [CO 2 ] – Controlling for temperature, he subjected plants to different [CO 2 ] – He found that photosynthesis decreased with a decreased [CO 2 ] CONCLUSIONS: 1.Light reactions occur in the presence of light, independent of temperature *use light & water to produce NADPH and ATP 2.Carbon fixation reactions rely on the energy of light reactions *dependent on temperature not light intensity

LIGHT and PLANTS Plants have photosynthetic pigments called photosystems – Found in the thylakoid membrane Photosystems absorb photons of specific wavelengths – Then, use light reactions to form ATP and NADPH in the stroma

LIGHT and PLANTS Action spectrum – illustrates the effectiveness with which different wavelengths of light promote photosynthesis Absorption spectrum – Illustrates how much light, at a specific wavelength, is absorbed by a specific molecule/substance (ex. chlorophyll a or b)

Chlorophyll & Accessory Pigments Chlorophyll a & b absorb photons with energies in the blue-violet and red regions – They reflect energies between 500nm – 600nm – Hence, why we see photosynthesizing organisms as green Chlorophyll a transfers light energy to carbon fixation reactions Chlorophyll b acts as an accessory pigment – Absorbs photons that chlorophyll a cannot

Chlorophyll & Accessory Pigments Carotenoids = accessory pigments – Ex. ß-Carotene – Enzymes split ß-Carotene into vitamin A – Vitamin A is used to help vertebrae vision in dim light environments – Carrots help vision as they have ß-Carotene Carotenoids absorb light energy from 400nm - 500nm (blue-violet) – Reflect yellow-red  appear yellow-orange Functions: Absorb energy that would damage chlorophyll, then give off energy as heat Protects human eye from excessive exposure to light

Why do leaves change colors in the fall? Leaves have accessory pigments that reflect multiple colors of light energy (red, yellow, orange) In spring/summer leafs appear green because they have [chlorophyll] in their chloroplasts of leaf cells – Remember: chlorophyll reflect green light! – Most likely temperature dependent In the fall, plants stop producing chlorophyll and dissemble any chlorophyll that is left in the leaves – Occurs due to the cooler temperatures This allos the reflected light of the accessory pigments to become visible!

PHOTOSYNTHETIC REACTION STAGES All photosynthetic reactions can be broken down into 3 distinct, but connected stages: 1.Capturing light energy 2.Using captured light energy to make ATP and reduced NADP+ **NADP+ is the photosynthetic equivalent of NAD+** 3.Using the free energy of ATP and the reducing power of NADPH to synthesize organic compounds (ex. creating glucose from CO 2 )